1. Field of the Invention
This invention relates to an acoustic panel that has at least one variable acoustic characteristic.
2. Description of Related Art
To limit the impact of sound pollution in the vicinity of airports, the international standards are increasingly restrictive as far as sound emissions are concerned.
Techniques have been developed to reduce the noise emitted by an aircraft, and in particular the noise emitted by an aircraft propulsion system, by using—at certain walls—coatings whose purpose is to absorb a portion of the sound energy, in particular by using the principle of Helmholtz resonators.
In a known way, this acoustic coating, also called an acoustic panel, comprises—from the outside to the inside—an acoustically resistive structure, an alveolar structure, and a reflective layer. Structure or layer is defined as one or more layers that may or may not be of the same type.
The acoustically resistive structure is a porous structure that plays a dissipative role, partially transforming the acoustic energy of the sound wave that passes through it into heat. It comprises so—called open zones that are able to allow acoustic waves to pass and other so—called closed or filled zones that do not allow the sound waves to pass but are designed to ensure the mechanical strength of said layer. This acoustically resistive layer is characterized primarily by an open surface ratio that is also called TSO.
In the case of a more complex acoustically resistive layer, for example the ones that comprise a metal material or carbon strips or more generally roughness at the surface in contact with the aerodynamic flows, other acoustic characteristics of the acoustically resistive layer can be adjusted, in particular its resistance to the flow at zero speed, also called RO, its non-linearity factor that is also called NLF.
One example of an acoustic panel is described in particular in the patent application FR-2,826,168 in the name of the applicant.
The acoustic panels are elements with localized reaction that can be characterized by their normal wall impedance. This impedance depends on numerous characteristics, in particular those of the acoustic panel, for example the height of the cavities of the alveolar structure, or more particularly the acoustically resistive structure (primarily TSO, RO, NLF). This impedance also depends on the characteristics that are linked to the air flow that flows at the surface of the panel and other characteristics that are linked to sound, in particular the frequency of the acoustic wave and its amplitude.
According to one application, an acoustic panel can be used to cover certain walls of a propulsion system, in particular those of a nacelle in which a power plant is placed in an essentially concentric manner.
The nacelle comprises an inside wall that delimits a duct with an air intake at the front, a first portion of the incoming air flow, called primary flow, passing through the power plant to participate in the combustion, whereby the second portion of air flow, called secondary flow, is conveyed by a fan and flows in an annular duct that is delimited by the inside wall of the nacelle and the outside wall of the power plant, whereby the different ducts have the same longitudinal axes.
To minimize the noise emitted by a propulsion system, the inside wall of the nacelle is coated by an acoustic panel that extends from the air intake to the rear of the secondary duct. The acoustic panel is generally made in several, preferably contiguous parts.
For a given nacelle and power plant, the characteristics of the acoustic panel and more particularly those of the acoustically resistive structure, in particular TSO, RO, and NLF, are determined so as to obtain an optimum impedance at the frequencies and engine speeds of interest so as to reduce as much as possible the noise that is emitted by the propulsion system in question.
Thus, for a propulsion system, i.e., for a given nacelle and power plant, the characteristics of the acoustic panel and more particularly those of the acoustically resistive structure, in particular TSO, RO, and NLF, are constant over the circumference and in the longitudinal direction of the nacelle.
This solution is not satisfactory because it does not make it possible to optimize the minimization of the noise along the duct of said nacelle.
Also, the purpose of this invention is to remedy the drawbacks of the prior art by proposing an acoustic panel that makes it possible to optimize the minimization of the noise throughout the propagation of the acoustic wave.